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Organization and Dynamics of Respiratory Electron Transport Complexes in Cyanobacteria

Final Report Summary - CYANOBAC-RESPIRATION (Organization and Dynamics of Respiratory Electron Transport Complexes in Cyanobacteria)

Life depends on the processes of energy conversion, in which energy obtained from sunlight, or from catalysing chemical reactions such as the breakdown of food molecules, is converted into energy in the forms that can be used to power the metabolism of the cell. Some key biological energy conversion processes take place in biological membranes and involves the transport of electrons from donors to acceptors, powered by either light (photosynthesis) or chemical energy (respiration). These processes require a set of membrane components including protein complexes and smaller molecules that can transport electrons in the membrane. The components involved are now well known, but their organisation and interactions in the intact membrane, as well as the balance of electron transport routes are much less understood. Electron transport and associated oxidative phosphorylation are important mechanisms for biological energy conversion, found in eukaryotic organelles (mitochondria and chloroplasts) and most bacteria. Electron transport routes are regulated by physiological factors, and have strong effects on the cell physiology. The pathways of electron transport depend largely on the organisation of the electron transport components in the membrane. However, it remains unclear on how electrons flow through the bacterium and on what length scales complexes might be organised in order to control electron transport pathways, and the factors that can 'stream' electrons between specific pairs of donor and acceptor complexes.

In this project, we use as our model organism a cyanobacterium, Synechococcus elongatus PCC7942, with both photosynthetic and respiratory electron transport pathways occurring in a complex membrane system inside the cell called the thylakoid membranes. With a combination of tagging with green fluorescent protein (GFP) and confocal fluorescence microscopy, we investigated the distribution and regulation of certain key electron-carrying complexes, i.e. type-I NAD(P)H dehydrogenase (NDH-1, Complex I) and succinate dehydrogenase (SDH, Complex II). Our study revealed that there is very significant lateral heterogeneity in the distribution of electron transport complexes in the thylakoid membranes. Furthermore, altering the environmental conditions of the bacterium, such as changing the light intensity, resulted in a dramatic redistribution of the complexes. When cells are grown under low light, both complexes are concentrated in discrete patches in the thylakoid membranes, about 100-300 nm in diameter and containing tens to hundreds of complexes. Exposure to moderate light leads to redistribution of both complexes such that they become evenly distributed within the thylakoid membranes. The effects of electron transport inhibitors strongly indicate that redistribution of respiratory complexes is actually triggered by changes in the redox state of an electron carrier close to the plastoquinone pool. We further showed that the distribution of these complexes on sub-micron scales is regulated according to physiological conditions. The redistribution of the complexes corresponds with a major change in electron direction and physiological balance of two electron transport routes.

This is the first study to visualise the dyanmic membrane organisation in controlling electron transport pathways in a bacterial membrane, and witness the 'biological electric switch' that dictates how electrons flow through the bacterium. We expect our studies on a cyanobacterium to act as an exemplar for studies of biological electron transport at the membrane scale in other systems, ranging from bacterial plasma membranes to the inner mitochondrial membranes, and to provide new ideas for the control of the organisation and function of biological membranes in general. It also suggests a new approach for controlling the biological energy conversion and cellular redox balance which could prove crucial for engineering organisms for enhanced biofuel production.